1. Field of the Invention
The present invention relates generally to an anti-lock or anti-skid braking-pressure control apparatus for a vehicle, and more particularly to the technique of improving the accuracy of control of the control apparatus.
2. Related Art Statement
There is known an anti-lock braking-pressure control apparatus which is used on a vehicle for reducing the braking distance, and improving the driving stability, upon application of a braking force by a driver to the brake system of the vehicle. An example of the conventional control apparatus is disclosed in laid-open Publication No. 1-182155 of unexamined Japanese Patent Application. The disclosed control apparatus includes (a) at least one wheel speed sensor which detects respective actual speeds of a plurality of wheels of a vehicle, (b) at least one solenoid-operated valve which regulates respective braking pressures applied to the wheels, and (c) a controller which electrically operates, for preventing locking of each of the wheels upon application of a braking force to the vehicle, the corresponding solenoid-operated valve based on a relationship between (1) the actual speed of the wheel in question detected by the corresponding wheel speed sensor and (2) a running speed of the vehicle.
The above-identified control apparatus utilizes the fact that the highest one of respective actual wheel speeds of a plurality of wheels of a vehicle approximates the running speed of the vehicle to a high degree. More specifically described, the vehicle speed is estimated based on the actual wheel speeds of the plurality of wheels, and an anti-lock braking-pressure regulating operation is carried out based on a relationship between the actual wheel speed of the wheel in question and the estimated vehicle speed. However, this technique suffers from limits to improvement of the accuracy of estimation of the vehicle speed, therefore, limits to improvement of the accuracy of control of the control apparatus.
In the above-described technical background, the Assignee of the present application had filed U.S. patent application Ser. No. 07/942,981, on Sep. 10, 1992 (U.S. Pat. No. 5,334,983 disclosing an anti-lock braking-pressure control apparatus which includes, in addition to the elements (a) to (c) of the above-identified first control apparatus, (d) a ground-speed sensor which detects a ground-speed of a vehicle as a running speed thereof relative to a road surface. In the disclosed control apparatus, the controller electrically operates, for regulating the braking pressure applied to a wheel in question, a corresponding solenoid-operated valve at a duty cycle controlled based on at least (i) a wheel speed error that is a deviation of the actual speed of the wheel from a target wheel speed obtained based on the ground-speed of the vehicle and (ii) a time derivative of the wheel speed error. More specifically, the disclosed control apparatus is designed such that the controller determines, at every time, the duty cycle of the solenoid-operated valve based on both of the two parameters (i) and (ii).
In the case of the above-described second control apparatus, however, since the duty cycle is determined whenever based on both the two parameters (i) and (ii), the control apparatus suffers from, for example, a tendency that the braking pressure applied to the wheel in question is excessively decreased. Thus, it cannot be said that the accuracy of control of the control apparatus is sufficiently high. That is, it is not advantageous that the only and same determining rule is used whenever for determining a duty cycle.